Electricity is transferred through electrical power systems at high-voltage levels of typically 15 kV to 500 kV. A power transformer steps up and down the voltages. A high-voltage bushing is the interface between the transformer winding and external power lines.
The high-voltage bushing is a hollow insulator through which a conductor passes. Each end of the conductor is connected to electrical equipment. For example, as noted above, the high-voltage bushing can be used to transition a power line into a transformer. The high-voltage bushing provides electrical isolation of power line conductors in transmission and distribution, substation, transformer, capacitor and power protection applications.
The top of the bushing is connected to the power line. There are several ways to connect the transformer winding to the external power lines with the high-voltage bushing. The winding cable can be connected to the bottom of the bushing conductor inside the transformer, which is a bottom connection. Alternatively, the winding cable (or “draw-lead cable) can be pulled through the inside of the bushing hollow tube and connected to the top of the bushing, which is a draw-lead connection. Another alternative is to partially replace the draw-lead cable with a rod.
To connect the draw-lead bushing to a transformer in the factory, an end of the draw-lead cable is connected to a draw-lead terminal. The bushing is lifted up with a crane vertically and lowered with its bottom close to, but still above, the transformer turret (opening). A long string or rope is usually dropped down through the bushing hollow tube and tightened to the draw-lead terminal. As the bushing is further lowered down into the transformer, the string is pulled up to guide the draw-lead cable through the bushing tube. A gasket, or o-ring, is disposed between the bushing flange and the transformer tank. The bushing flange is then bolted to the transformer turret, and the draw-lead terminal is connected to the top of the bushing. The top of the bushing (such as a top terminal) is then connected to the external power line or testing cable.
To remove the high-voltage bushing for shipment of the transformer, the draw-lead terminal is disconnected from the bushing top, the draw-lead cable lowered into the transformer, the bushing flange unbolted, and the bushing removed from the transformer. The same installation process described above is followed to connect the high-voltage bushing in the field, except that the draw-lead cable is already connected to the draw-lead terminal such that the installer does not need to be inside the transformer.
In a draw-lead bushing, the draw-lead cable or draw-lead rod has full contact with the bushing top, either directly to the top of the bushing tube or through a conductor to the top of the bushing tube. However, the bottom of the draw-lead cable or rod does not have full contact with the bushing tube, especially when the draw-lead cable is insulated.
One problem associated with a high-voltage bushing is spark-over, or arcing, between the draw-lead cable conductor and the inner surface of the hollow tube of the bushing through which the conductor passes, thereby resulting in failure of the bushing. The spark-over can be caused by a high-frequency, or fast front, transient. Two causes of high-frequency transients are lightning induced insulator spark-overs and circuit switching. The surge wave created by the high-frequency transient travels to the bushing top and splits into the bushing tube and the draw-lead cable. The split waves travel along the two separate paths with different speeds and reflect differently, thereby creating a large potential difference between the adjacent positions on the bushing tube and the draw-lead cable. The large potential difference results in the arcing or spark-over between the two bushing tube and the draw-lead cable, thereby resulting in failure of the high-voltage bushing. These failures are often explosive and can result in fires. Additional insulation has been added to the draw-lead cable to prevent spark-over, but has not proved effective.
A conventional draw-lead cable bushing 11 is shown in FIGS. 1 and 2. A flange 13 connects the bushing 11 to electrical equipment, such as a transformer 111 (FIG. 3). A housing 15 is connected to the flange 13 and is exposed externally of the electrical equipment to which the bushing 11 is connected. A hollow bushing tube 17 passes through the bushing 11 from a first end 16 to a second end 18. A draw-lead conductor 21 is connected at a first end to the transformer windings disposed within the core 113 (FIG. 3). A draw-lead terminal 23 is connected to a second end of the draw-lead cable 21. The rope 25 is connected to draw-lead terminal 23 to guide the draw-lead cable through the bushing tube 17. A locking pin 27 is passed through the bushing tube 17 and the draw-lead terminal 23 to secure the draw-lead cable 21 to the first end 16, or top, of the bushing 11. The rope 25 is then disconnected from the draw-lead terminal 23, as shown in FIG. 2. As shown in FIG. 2, the draw-lead cable 21 has electrical contact with the bushing tube 17 at the upper end of the assembled high-voltage bushing 11. However, there is no electrical contact at the lower end of the bushing 11. The potential difference between the inner surface 29 of the bushing tube 17 and the outer surface 31 of the draw-lead cable 21 caused by a high-frequency transient surge wave can cause a spark-over between the bushing tube and cable, thereby resulting in failure of the high-voltage bushing 11.
Thus, there is a continuing need to provide a spark-over prevention device for a high-voltage bushing.